Javascript/Errors

Lesson 6 of 6

Chapter 9 Cheatsheet: Errors

This cheatsheet summarizes how to generate and handle runtime errors in JavaScript covered in Chapter 9.

The Error Object

Use the new Error() constructor to generate standard error objects. They contain a .message property and automatically capture a stack trace.

const err = new Error("Invalid configuration");
console.log(err.message); // "Invalid configuration"

Throwing Errors

Use the throw keyword to intentionally stop execution and trigger an error state. Always throw an Error object, never a plain string.

function checkStock(count) {
  if (count <= 0) {
    throw new Error("Out of stock");
  }
}

try / catch / finally

  • try: Wraps dangerous code. If an error is thrown anywhere inside, execution halts and jumps to catch.
  • catch(err): Captures the thrown error so you can log it or recover without crashing the program.
  • finally: (Optional) Code placed here is guaranteed to execute regardless of whether an error occurred. Useful for closing database connections or cleaning up resources.
try {
  // Dangerous operation
  const data = fetchFromNetwork();
} catch (err) {
  // Graceful fallback
  console.log("Network failed:", err.message);
} finally {
  // Guaranteed execution
  console.log("Cleanup complete.");
}

Best Practices

  • Only use try/catch for handling unpredictable data (APIs, user input) or recovering from specific failures.
  • Do not use try/catch to hide poorly written code or silence standard undefined TypeErrors. Fix the bug or use Optional Chaining (?.) instead.